1. Field of the Invention
The invention relates to thermotropic aromatic polyesters based on phenoxyterephthalic acid, another aromatic dicarboxylic acid, at least one aromatic hydroxycarboxylic acid, and at least one dihydroxyaryl compound.
2. Discussion of the Background
Aromatic polyesters have been known for a long time, and are regarded as high-temperature-stabile materials. The manufacture and use of such materials is described in the following literature, as an example: Eareckson, W. M., 1959, J. Polym. Sci., XL, 399; Bier, G., 1974, Polymer, 15, 527; Ciferri, A., Krigbaum, W. R., and Meyer, R. B., 1982, "Polymer Liquid Crystals", pub. Academic press, New York; and Erdemir, A. G., Johnson, D. J., and Tomka, J. G., 1986, Polymer, 27, 441.
Ger. OS No. 20 25 971 describes high molecular weight thermotropic polyesters based on aromatic dicarboxylic acids such as, e.g., terephthalic and isophthalic acids, dihydroxyaryl compounds such as, e.g., hydroquinone and 4,4'-dihydroxybiphenyl, and p-hydroxybenzoic acid. All these polyesters are difficult to process.
Copolyesters comprised of isophthalic acid and hydroquinone units to the extent of at least 80 mol % are described in Eur. OS No. 0 045 499. These polyesters can be molded or extruded, etc., in the melt.
A number of aromatic polyesters have excellent thermal stability, however their processing temperatures are so high that the polymer becomes colored and is partially decomposed. Problems also occur in the forming of the polyesters where there are high shear forces. In particular, the percentage of shrinkage in the processing direction differs markedly from that in the direction transverse to the processing direction. This problem is solved by polycondensing aromatic dicarboxylic acids and dihydroxyaryl compounds in the presence of a trifunctional compound, e.g., a dihydroxyaromatic monocarboxylic acid.
U.S. Pat. No.3,723,388 describes polyesters based on dihydroxyaryl compounds (bisphenols) and a mixture of at least 10 mol % of a substituted phenoxyphthalic acid and 0-90 mol % of another aromatic dicarboxylic acid or a derivative of same. The underlying concept of U.S. Pat. No. 3,723,388 is that polyesters based on phenoxyphthalic acids can be processed at much lower temperatures than the corresponding esters of unsubstituted phthalic acids. For example, in the patent noted above there is a reference to the noncrystalline properties of a polyester based on phenoxyterephthalic acid and hydroquinone (see Example 5), but nothing is mentioned as to possible liquid crystalline properties.
Jap. No. 60/69,312 describes linear aromatic polyesters prepared from phenoxyterephthalic acid and bisphenols and having an inherent viscosity of at least 1.0. The phenoxyterephthalic acid in these polyesters optionally can be replaced to the extent of up to 20 mol %, preferably only up to 10 mol %, by other aromatic dicarboxylic acids. However, it is important that the carboxyl groups of these dicarboxylic acids are oppositely directed and parallel to each other, as is the case, e.g., in terephthalic acid. Also, the hydroxyl groups of the dihydroxyaryl compounds should be oppositely directed. While the possibility is mentioned of including hydroxycarboxylic acids in the monomer component, there is no experimental confirmation.
These polyesters are prepared in the presence of nonreactive (i.e., not participating in the reaction), high boiling compounds with molecular weight &lt;1,000, required to reach the desired degree of polymerization of the polyesters and to avoid a decomposition of the polyesters at temperatures above the melting temperature. Following polycondensation and forming, the low molecular weight added agents must be extracted out of the finished molded (or otherwise formed) piece. This extraction process step, which may take up to 1 hr, is industrially extremely disadvantageous.
In Comparison Example 1 of Japanese No. 60/69,312 the preparation of a polyester by reacting phenoxyterephthalic acid diphenyl ester and hydroquinone is described. The polyester has a melting point of 320.degree. C., but is first flowable at 405.degree. C., at which temperature it decomposes. A specific statement is made that this polyester cannot be processed at temperatures below 400.degree. C.
Thus, from the state of the art, aromatic polyesters based on terephthalic acid, hydroquinone, and p-hydroxybenzoic acid are known, as are aromatic polyesters based on phenoxyterephthalic acid and dihydroxyaryl compounds. However, liquid crystalline polyesters based on phenoxyterephthalic acid, one or more other dicarboxylic acids, dihydroxyaryl compounds, and aromatic hydroxycarboxylic acids are not yet known.